Water beds are known whose principle consists in placing on a rigid undermattress, generally formed by a frame whose bottom is closed by a wooden plank, a water mattress formed of a sealed material envelope filled with water. This type of bed has the drawback of requiring a sufficient thickness of water so that the user does not rest directly on the wooden support. Because of this thickness, this type of bed has considerable weight and high thermal inertia.
To overcome this latter drawback, a system is sometimes added for heating the water so that the user does not have the unpleasant feeling of a cold contact. On the other hand, this bed has the advantage of uniformly distributing the pressure of the water over the whole body. These beds are used for this advantage, particularly in medicine.
Because of the direct contact of the sealed envelope with the body of the user, problems arise due to condensation and the lack of aeration for the part of the body of the user in contact with the sealed envelope.